Image processing apparatus and image processing method for performing correction processing on input video

ABSTRACT

An image processing apparatus according to the present invention extracts a characteristic value of a luminance in relation to respective fields of an input video, and determines the presence of a scene change between adjacent fields. A gamma curve is then generated on the basis of the magnitude of the characteristic value. When a difference in the characteristic value between fields is larger than a predetermined value and a scene change does not exist, the gamma curve to be applied to a subsequent field is modified such that the correction characteristic of the gamma curve does not vary rapidly. The luminance is then corrected using the modified gamma curve.

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/764,384,filed on Apr. 21, 2010. It claims benefit of that application under 35U.S.C. §120, and claims benefit under 35 U.S.C. §119 of Japanese PatentApplication No. 2009-104776, filed on Apr. 23, 2009. The entire contentsof each of the mentioned prior applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing apparatus and animage processing method for performing correction processing on an inputvideo.

2. Description of Related Art

In a known technique for improving the image quality of a displayedvideo, a gamma characteristic of an input video is modified by detectingcharacteristics of the input video. A technique of this type isdescribed in Japanese Patent Application Publication No. 2001-169143,for example. In another known technique, a gamma curve representing thegamma characteristic is generated dynamically. A technique of this typeis described in Japanese Patent Application Publication No. 2006-319953,for example. Normally, measures are taken during this type of processingto modify the gamma characteristic gently to prevent viewer discomfortwhen a contrast and so on of the input video varies rapidly.

Meanwhile, a cinema video of a motion picture is constituted by 24frames per second and is converted into a 60 frames-per-secondtelevision signal using 2-3 pulldown processing. In another knowntechnique, appropriate image processing is performed when a televisionsignal receiver determines that an input video has undergone 2-3pulldown processing, and as a result, a high-quality cinema video isdisplayed. A technique of this type is described in Japanese PatentApplication Publication No. 2007-74439, for example.

Further, a video display apparatus that displays a video having a largernumber of frames than the number of frames of an input video using framerate conversion processing exists. For example, when a 2-3pulldown-processed video is displayed on an apparatus for displaying a120 frames-per-second video, a 120 frames-per-second display can berealized by displaying each frame of the cinema video five timesconsecutively (24 frames per second□5=120 frames). A technique fordisplaying a high-quality cinema video in this manner is described inJapanese Patent Application Publication No. 2004-302045, for example.

SUMMARY OF THE INVENTION

However, when a 2-3 pulldown-processed video is displayed or when eachframe of a cinema video is displayed five times, the same frame isdisplayed repeatedly. Meanwhile, when a method of displaying the sameframe twice consecutively is employed during frame rate conversion toconvert a 60 Hz video into 120 Hz, the same frame is likewise displayedrepeatedly. Processing for suppressing rapid variation in the gammacharacteristic, as described above, is conventionally performed in thesecases also. Accordingly, processing is performed to vary the gammacharacteristic gently even when an image does not vary between frames.As a result, it becomes impossible to display the same image repeatedlywith accuracy, and therefore the original video cannot be reproduced.

Furthermore, conventionally, processing to suppress rapid variation inthe gamma characteristic is performed without taking into considerationthe presence of a scene change. However, in the case of a scene change,a user does not experience a sense of discomfort even when the gammacharacteristic varies rapidly. When variation is suppressed regardless,a reproduction quality of the original video deteriorates, leading tophenomena such as a loss of sharpness in the video.

The present invention has been designed in consideration of thesecircumstances, and an object thereof is to provide an image processingapparatus or method which, during correction processing of an inputvideo, applies a suitable gamma curve for reproducing an original video,taking into account the presence of a scene change, frame rateconversion processing, and so on.

An image processing apparatus according to this invention is that,

an image processing apparatus comprising:

a characteristic extraction unit that extracts a characteristic value ofa luminance in relation to respective input images constituting an inputvideo and detects the presence of a scene change between adjacent inputimages;

a control unit that determines a luminance correction curve to beapplied to output images corresponding to the respective input images onthe basis of a magnitude of the characteristic value extracted by thecharacteristic extraction unit; and

a luminance correction unit that corrects a luminance of the respectiveoutput images, using the luminance correction curve determined by thecontrol unit,

wherein, when a difference in the characteristic value between a firstinput image and a subsequent second input image is larger than apredetermined value and a scene change does not exist between the firstinput image and the second input image, the control unit determines aluminance correction curve having an intermediate correctioncharacteristic between a luminance correction curve corresponding to acharacteristic value of the first input image and a luminance correctioncurve corresponding to a characteristic value of the second input imageas a luminance correction curve to be applied to an output imagecorresponding to the second input image, and

when the difference in the characteristic value is smaller than thepredetermined value or a scene change exists between the first inputimage and the second input image, the control unit determines theluminance correction curve corresponding to the characteristic value ofthe second input image as the luminance correction curve to be appliedto the output image corresponding to the second input image.

An image processing method according to this invention is that,

an image processing method comprising the steps of:

extracting a characteristic value of a luminance in relation torespective input images constituting an input video;

detecting the presence of a scene change between adjacent input images;

determining a luminance correction curve to be applied to output imagescorresponding to the respective input images on the basis of a magnitudeof the extracted characteristic value; and

correcting a luminance of the respective output images using thedetermined luminance correction curve,

wherein, when a difference in the characteristic value between a firstinput image and a subsequent second input image is larger than apredetermined value and a scene change does not exist between the firstinput image and the second input image, a luminance correction curvehaving an intermediate correction characteristic between a luminancecorrection curve corresponding to a characteristic value of the firstinput image and a luminance correction curve corresponding to acharacteristic value of the second input image is determined in thedetermining step as a luminance correction curve to be applied to anoutput image corresponding to the second input image, and

when the difference in the characteristic value is smaller than thepredetermined value or a scene change exists between the first inputimage and the second input image, the luminance correction curvecorresponding to the characteristic value of the second input image isdetermined in the determining step as the luminance correction curve tobe applied to the output image corresponding to the second input image.

According to the present invention, during correction processing of aninput video, a suitable gamma curve for reproducing an original videocan be applied, taking into account the presence of a scene change,frame rate conversion processing, and so on.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the constitution of an imageprocessing apparatus;

FIG. 2 is a view showing an example of a gamma curve;

FIG. 3A is a view showing an output timing from a characteristicextraction unit according to a first embodiment, while FIGS. 3B and 3Care views showing examples of an applied gamma instruction valueaccording to the first embodiment;

FIG. 4 is a flowchart illustrating gamma correction in a case whereframe rate conversion is not performed;

FIGS. 5A to 5C are views showing examples of an applied gammainstruction value according to a second embodiment;

FIG. 6A is a view showing the constitution of an input video accordingto a third embodiment, while FIGS. 6B and 6C are views showing examplesof an applied gamma instruction value according to the third embodiment;and

FIG. 7A is a view showing the constitution of an input video accordingto a fourth embodiment, while FIGS. 7B and 7C are views showing examplesof an applied gamma instruction value according to the fourthembodiment.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will be described indetail below with reference to the drawings.

First Embodiment

The constitution of an image processing apparatus will now be describedusing the block diagram shown in FIG. 1. The image processing apparatusincludes blocks constituted by a characteristic extraction unit 101, apulldown signal detection unit 102, a interlace-progressive conversionunit 103, a frame rate conversion unit 104, a gamma correction unit 105,a frame rate conversion setting unit 106, and a gamma correction controlunit 107. In the drawing, thick arrows indicate the course of an inputvideo as it passes through various processes to become an output video,while thin arrows indicate the flow of control data and so on. The imageprocessing apparatus according to the present invention functions toextract an input video from a received broadcast wave, implement imageimprovement on the extracted input video, and then output an outputvideo to a display on which the output video is displayed. For example,the image processing apparatus may be incorporated into a televisionapparatus including a display, or may be an STB (Set Top Box) or thelike that outputs a video to a display.

The characteristic extraction unit 101 calculates a characteristic valuefor each field of the input video. The characteristic value is output tothe gamma correction control unit 107. A total amount value of aluminance of pixels included in the field is used as the characteristicvalue. The characteristic extraction unit also detects whether or not ascene change occurs between adjacent fields of the input video andoutputs a determination result to the gamma correction control unit. Inthis embodiment, average luminance values of two consecutive fields (afirst input image and a second input image) are compared, and when adifference is larger than a predetermined value, a scene change isdetermined to be present.

The pulldown signal detection unit 102 determines whether or not theinput video has undergone 2-3 pulldown processing and outputs a resultto the gamma correction control unit. A method of determining thepresence of two or three consecutive fields created from an identicalframe or a method of detecting a recognition signal indicating 2-3pulldown processing, which has been embedded on a broadcast transmissionside, may be used as a determination method. When the input video is aninterlace signal generated by interlace scanning, theinterlace-progressive conversion unit 103 performs interlace-progressiveconversion as required to create a progressive signal. The frame rateconversion unit 104 modifies a number of displayed images per secondusing a conversion method instructed by the frame rate conversionsetting unit 106. The frame rate conversion setting unit 106 sets aframe rate conversion method in accordance with a user operation or thelike, and outputs a setting content to the frame rate conversion unitand the gamma correction control unit. The setting content includes amethod of creating an intermediate image, a method of repeating anidentical image, and so on such that a plurality of output images areassociated with a field (or a frame) of the input video throughrespectively different methods.

The gamma correction unit 105 performs non-linear conversion processingon the input video using a gamma curve selected by the gamma correctioncontrol unit. The gamma correction control unit 107 selects an optimumgamma curve using the characteristic value and the presence of a scenechange from the characteristic extraction unit, the determination resultfrom the pulldown signal detection unit, and the setting content fromthe frame rate conversion setting unit, and outputs the selected gammacurve to the gamma correction unit. Examples of gamma curves areindicated by encircled numerals 1 to 9 in FIG. 2. In the drawing, theabscissa shows input gradations and the ordinate shows outputgradations. The gamma correction control unit classifies the magnitudeof the characteristic value according to nine gradations and selects anyone of the gamma curves indicated by the encircled numerals 1 to 9corresponding to the respective gradations. For example, when thecharacteristic value corresponds to the largest gradation, or in otherwords when the image has a maximum brightness level, the gamma curve 9is selected to prevent highlight detail loss. Conversely, when thecharacteristic value corresponds to the smallest gradation, or in otherwords when the image is dark, the gamma curve 1 is selected to preventshadow detail loss. When the characteristic value takes an intermediatevalue, one of the gamma curves 2 to 8 is selected in accordance with thevalue.

Note that the gamma correction unit corresponds to a luminancecorrection unit of the present invention. Further, the gamma curvecorresponds to a luminance correction curve of the present invention,and although a so-called gamma curve is used in the embodiments, theluminance correction curve is not limited to a gamma curve as long asthe luminance of the input image can be converted into the luminance ofthe output image. Further, the gamma correction control unit correspondsto a control unit of the present invention. Furthermore, in theembodiments, a method of selecting one of a plurality of gamma curves isused, but a gamma curve may be generated dynamically on the basis of thecharacteristic value of the input image. Alternatively, an appropriategamma curve may be generated using a plurality of gamma curves.

In this embodiment, the input video is an interlace signal having afield frequency of 60 Hz, i.e. constituted by 60 fields per second. Inother words, respective input images (fields) constituting the inputvideo are switched at a rate of 60 images per second. Further, 2-3pulldown processing is not implemented on the input video. Furthermore,the input video is subjected to interlace-progressive conversion.Moreover, an instruction indicating that “Frame rate conversion is notto be performed” is issued to the frame rate conversion setting unit bythe user. In consideration of these prerequisites, a method employed bythe gamma correction control unit to select a gamma curve and implementgamma correction will now be described.

FIG. 3A shows a timing at which the characteristic extraction unitoutputs the results of characteristic value calculation and scene changedetection from the input video. As shown in the drawing, the results areoutput at a timing immediately before the end of a field. The gammacorrection control unit selects a gamma curve on the basis of the outputresults and so on. The gamma correction unit then performs non-linearconversion processing based on the selected gamma curve to obtain anoutput video. Note that gamma curve selection by the gamma correctioncontrol unit is performed specifically by selecting an applied gammainstruction value. The applied gamma instruction value is a valuespecifying one of the gamma curves shown in FIG. 2, and is thereforerepresented by a numerical value between 1 and 9. As described above,the applied gamma instruction value increases as the characteristicvalue (the total amount value of the luminance, or in other words thebrightness of the image) increases.

FIG. 3B shows an applied gamma instruction value selected by the gammacorrection control unit when a scene change does not occur and thecharacteristic value decreases greatly from a field A to a field B butdoes not vary from the field B to a field C. During the transition fromthe field A to the field B, the characteristic value decreases greatly,and when the applied gamma instruction value is set at a valuecorresponding to the characteristic value variation, the output imagemay suddenly become too bright, causing user discomfort. Therefore, asshown in FIG. 3B, the applied gamma instruction value applied to theoutput frame B is set at an intermediate value between the value appliedto the output frame A and the original value corresponding to thecharacteristic value. When the amount by which the luminance of theoutput video varies is controlled in this manner, the gamma curve can bemodified in frame units in a stepped fashion. As a result, an outputvideo that does not cause user discomfort can be displayed.

FIG. 3C shows an applied gamma instruction value selected by the gammacorrection control unit when a scene change is detected between thefields A and B and the characteristic value decreases greatly from thefield A to the field B but does not vary from the field B to the fieldC. In this case, a scene change occurs between the output frames A andB, and therefore the user does not experience a sense of discomfort evenwhen the brightness of the images varies rapidly. Hence, the gammacorrection control unit selects the applied gamma instruction valuecorresponding to the characteristic value as is, and then causes thegamma correction unit to perform correction. In a case wherecharacteristic value variation is caused by a scene change, the originalvideo cannot be reproduced when the gamma curve is varied in a steppedfashion, and therefore the sharpness of the displayed video may be lost.Hence, when a scene change is detected, gamma correction correspondingdirectly to the characteristic value is performed as described above,and as a result, favorable video display in which the reproducibility ofthe original video is high can be achieved.

The processing of FIGS. 3B and 3C will now be described further using aflowchart shown in FIG. 4. In a step S01, the characteristic extractionunit calculates a characteristic value VA and an average luminance valueof the field A, and the gamma correction control unit selects an appliedgamma instruction value P corresponding to VA. In a step S02, thecharacteristic extraction unit calculates a characteristic value VB andan average luminance value of the field B, and the gamma correctioncontrol unit selects an applied gamma instruction value R correspondingto VB. The applied gamma instruction values P and R are shown in FIGS.3B and 3C, respectively. In a step S03, the characteristic extractionunit compares the average luminance values of the fields A and B todetermine the presence of a scene change. In a step S04, the gammacorrection control unit compares VA and VB to determine whether or not adifference between the two is larger than a predetermined value VT. WhenS04=NO is established, the gamma correction control unit confirmsselection of the applied gamma instruction value R corresponding to thefield B. When S04=YES is established, on the other hand, the gammacorrection control unit decides to perform subsequent processing basedon the presence of a scene change in a step S05. When S05=YES isestablished, the likelihood that the characteristic value variation iscaused by a scene change is high, and therefore the gamma correctioncontrol unit confirms selection of the applied gamma instruction value Rcorresponding to the field B. When S05=NO is established, on the otherhand, the gamma correction control unit modifies the applied gammainstruction value to Q in order to suppress rapid variation of the gammacurve.

Note that an average luminance value of the pixels included in the imagemay be used as the characteristic value of the image. Further, aluminance histogram having a luminance range on the abscissa and a pixelcount on the ordinate may be created and the gamma curve may be selectedon the basis of the shape of the histogram (on the basis of a peak valuewhen the histogram can be approximated by a normal distribution, forexample).

Second Embodiment

In this embodiment, the input video is an interlace signal having afield frequency of 60 Hz. Further, 2-3 pulldown processing is notimplemented on the input video. Furthermore, the input video issubjected to interlace-progressive conversion. Moreover, an instructionindicating that “The signal is to be converted into a signal having aframe frequency of 120 Hz” is issued to the frame rate conversionsetting unit by the user. In consideration of these prerequisites, amethod employed by the gamma correction control unit to select a gammacurve and implement gamma correction will now be described. Note thatthe apparatus constitution is identical to the first embodiment.

In this embodiment also, the characteristic extraction unit outputsresults relating to calculation of the characteristic value and thepresence of a scene change at the timing indicated in FIG. 3A. The framerate conversion unit then creates an interpolation frame as aninterpolation image in accordance with the setting content of the framerate conversion setting unit, and inserts the interpolation frame into aframe boundary of the output video.

FIG. 5A shows the applied gamma instruction value when a scene changedoes not occur and the characteristic value decreases greatly from thefield A to the field B but does not vary from the field B to the fieldC. In this drawing, frame rate conversion is performed by the frame rateconversion unit using a method of creating an intermediate image frompreceding and following frames. In other words, an intermediate imageA′, an intermediate image B′, and an intermediate image C′ are generatedfrom frames A and B, frames B and C, and frame C and a subsequent frame,respectively. This method corresponds to a second interpolation methodof the present invention. Here, the gamma correction control unit canperform detailed image processing by selecting an applied gammainstruction value for a video that has already undergone frame rateconversion. For example, an intermediate value between the applied gammainstruction values of the frames A and B is selected in relation to theintermediate image A′. As a result, the gamma characteristic can bemodified smoothly without causing viewer discomfort.

Meanwhile, FIG. 5B shows the applied gamma instruction value in a casewhere the frame rate conversion unit performs frame rate conversionusing a method of repeating a preceding frame. This method correspondsto a first interpolation method of the present invention. As shown inthe drawing, the gamma correction control unit selects an identicalapplied gamma instruction value in relation to identical images. In sodoing, the reproducibility of the original video can be improved. Byvarying the applied gamma instruction value in a stepped fashion takinginto consideration the frame rate conversion method in this manner,rapid brightness variation can be suppressed and the original video canbe reproduced.

FIG. 5C shows the applied gamma instruction value selected by the gammacorrection control unit when a scene change is detected between thefields A and B and the characteristic value decreases greatly from thefield A to the field B but does not vary from the field B to the fieldC. In this case, a scene change occurs, and therefore the user does notexperience a sense of discomfort even when the image brightness variesrapidly. Hence, the gamma correction control unit selects the appliedgamma instruction value corresponding to the characteristic value as is.

Thus, gamma correction control can be performed appropriately by thegamma correction control unit in accordance with the characteristicvalue, the presence of a scene change, and the frame rate conversionmethod even when the frame rates of the input video and the displayedvideo differ, and as a result, a favorable gamma characteristic can beset.

Third Embodiment

In this embodiment, the input video is an interlace signal having afield frequency of 60 Hz. Further, 2-3 pulldown processing isimplemented on the input video. Furthermore, the input video is notsubjected to interlace-progressive conversion. Moreover, an instructionindicating that “The signal is to be converted into a signal having aframe frequency of 120 Hz” is issued to the frame rate conversionsetting unit by the user. In consideration of these prerequisites, amethod employed by the gamma correction control unit to select a gammacurve and implement gamma correction will now be described. Note thatthe apparatus constitution is identical to the first embodiment. Notealso that although interlace-progressive conversion is not performed inthis embodiment, processing is identical to a case in whichinterlace-progressive conversion is performed.

FIG. 6A shows the constitution of an input video subjected to 2-3pulldown processing. As shown in the drawing, an image A is displayedtwice consecutively, an image B is displayed three times consecutively,an image C is displayed twice consecutively, and an image D is displayedthree times consecutively. Hence, in the 2-3 pulldown processing, aframe rate difference is absorbed by creating two or three fields from asingle frame of a cinema video. When the frame rate conversion unitconverts an input video such as that shown in FIG. 6A into framefrequency 120 Hz, a single frame of the original cinema video isdisplayed five times consecutively, as shown in FIG. 6B. With thismethod, a video can be displayed at an identical timing to the originalcinema video.

FIG. 6B shows the applied gamma instruction value selected by the gammacorrection control unit when a scene change does not occur and thecharacteristic value decreases greatly from the field A to the field Bbut does not vary from the field B to a field D. As shown in thedrawing, an identical applied gamma instruction value is used for framescreated from an identical frame of the cinema video. Thus, ahigh-quality video that faithfully reproduces the original cinema videocan be displayed. Furthermore, in this embodiment, a scene change doesnot occur, and therefore the gamma characteristic is varied graduallyfrom the frame B to the frame D. In so doing, a situation in which theuser experiences a sense of discomfort due to rapid variation in thebrightness of the screen can be avoided.

FIG. 6C shows the applied gamma instruction value selected by the gammacorrection control unit when a scene change is detected between thefields A and B and the characteristic value decreases greatly from thefield A to the field B but does not vary from the field B to the fieldD. In this case, a scene change occurs, and therefore the user does notexperience a sense of discomfort even when the gamma characteristicvaries rapidly. Hence, the gamma correction control unit applies thegamma characteristic corresponding to the characteristic value of thefield B as is. Meanwhile, an identical applied gamma instruction valueis used for frames created from an identical frame of the cinema video,and therefore the original video can be displayed with a high degree ofreproducibility.

Hence, the gamma correction control unit can perform gamma correctioncontrol appropriately in accordance with the characteristic value andthe presence of a scene change even when the input video is cinema videosubjected to 2-3 pulldown processing such that the frame rate isconverted. As a result, gamma characteristic modification suitable for a24 frame-per-second cinema video can be performed, enabling faithfulreproduction of the cinema video.

Fourth Embodiment

In this embodiment, the input video is an interlace signal having afield frequency of 60 Hz. Further, 2-3 pulldown processing isimplemented on the input video. Furthermore, the input video is notsubjected to interlace-progressive conversion. Moreover, an instructionindicating that “Frame rate conversion is not to be performed” is issuedto the frame rate conversion setting unit by the user. In considerationof these prerequisites, a method employed by the gamma correctioncontrol unit to select a gamma curve and implement gamma correction willnow be described. Note that the apparatus constitution is identical tothe first embodiment. Note also that the constitution of the input videoshown in FIG. 7A is identical to that of the third embodiment describedabove using FIG. 6A.

FIG. 7B shows the applied gamma instruction value selected by the gammacorrection control unit when a scene change does not occur and thecharacteristic value decreases greatly from the field A to the field Bbut does not vary from the field B to the field D. As shown in thedrawing, an identical applied gamma instruction value is used for framescreated from an identical frame of the cinema video. Thus, ahigh-quality video that faithfully reproduces the original cinema videocan be displayed. Furthermore, in this embodiment, a scene change doesnot occur, and therefore the gamma characteristic is varied graduallyfrom the frame B to the frame D. In so doing, a situation in which theuser experiences a sense of discomfort due to rapid variation in thebrightness of the screen can be avoided.

FIG. 7C shows the applied gamma instruction value selected by the gammacorrection control unit when a scene change is detected between thefields B and C and the characteristic value decreases greatly from thefield B to the field C but does not vary on other field boundaries. Inthis case, a scene change occurs, and therefore the user does notexperience a sense of discomfort even when the gamma characteristicvaries rapidly. Hence, the gamma correction control unit selects theapplied gamma instruction value such that a gamma characteristiccorresponding to the characteristic value of the field C is applied asis. Meanwhile, an identical applied gamma instruction value is used forframes created from an identical frame of the cinema video, andtherefore the original video can be displayed with a high degree ofreproducibility.

Hence, the gamma correction control unit can perform gamma correctioncontrol appropriately in accordance with the frame rate conversionmethod, the characteristic value, the presence of a scene change, andthe pulldown detection result even when the input video is cinema videosubjected to 2-3 pulldown processing. As a result, a 24 frame-per-secondcinema video can be reproduced even more faithfully, and gammamodification can be performed favorably.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. An image processing apparatus comprising: acharacteristic extraction unit that extracts a characteristic value of aluminance in relation to respective input images constituting an inputvideo and detects a presence of a scene change in the input video; acontrol unit that determines a gamma correction curve to be applied tooutput images corresponding to the respective input images on the basisof a magnitude of the characteristic value extracted by saidcharacteristic extraction unit; and a gamma correction unit thatcorrects a luminance of the respective output images using the gammacorrection curve determined by said control unit, wherein, when adifference in the characteristic value between a first input image and asubsequent second input image is larger than a predetermined value and ascene change does not exist between the first input image and the secondinput image, said control unit determines a gamma correction curve to beapplied to the output image corresponding to the second input image soas to suppress change of a gamma curve, and when the difference in thecharacteristic value between the first input image and the subsequentsecond input image is equal to or smaller than the predetermined value,said control unit determines the gamma correction curve corresponding tothe characteristic value of the second input image as the gammacorrection curve to be applied to the output image corresponding to thesecond input image.
 2. The image processing apparatus according to claim1, wherein when the difference in the characteristic value between thefirst input image and the subsequent second input image is larger thanthe predetermined value and a scene change exists between the firstinput image and the second input image, said control unit determines thegamma correction curve corresponding to the characteristic value of thesecond input image as the gamma correction curve to be applied to theoutput image corresponding to the second input image.
 3. The imageprocessing apparatus according to claim 1, wherein when the differencein the characteristic value between the first input image and thesubsequent second input image is larger than a predetermined value and ascene change does not exist between the first input image and the secondinput image, said control unit determines a gamma correction curvehaving a correction characteristic between a gamma correction curvecorresponding to a characteristic value of the first input image and agamma correction curve corresponding to a characteristic value of thesecond input image as the gamma correction curve to be applied to anoutput image corresponding to the second input image.
 4. The imageprocessing apparatus according to claim 3, wherein when the differencein the characteristic value between the first input image and thesubsequent second input image is larger than a predetermined value and ascene change does not exist between the first input image and the secondinput image, said control unit determines a gamma correction curvehaving an intermediate correction characteristic between a gammacorrection curve corresponding to a characteristic value of the firstinput image and a gamma correction curve corresponding to acharacteristic value of the second input image as the gamma correctioncurve to be applied to an output image corresponding to the second inputimage.
 5. The image processing apparatus according to claim 1, whereinthe characteristic value is a total amount value of a luminance of aplurality of pixels included in the input image.
 6. The image processingapparatus according to claim 1, further comprising a frame rateconversion unit that inserts an interpolation image created using eithera first interpolation method or a second interpolation method betweenadjacent input images, wherein, in the first interpolation method, theinterpolation image is created by repeating an immediately precedingimage and in the second interpolation method, the interpolation image iscreated from preceding and following images, and said control unitdetermines a gamma correction curve to be applied to the interpolationimage in accordance with the interpolation method used to create theinterpolation image.
 7. The image processing apparatus according toclaim 6, wherein, when the interpolation image is created using thefirst interpolation method, said control unit applies to theinterpolation image an identical gamma correction curve to that of theimmediately preceding image, and when the interpolation image is createdusing the second interpolation method, said control unit determines thegamma correction curve to be applied to the interpolation image on thebasis of gamma correction curves applied to the preceding and followingimages.
 8. The image processing apparatus according to claim 1, furthercomprising a pulldown signal detection unit that determines whether ornot the input video is a video created from a 24 frame-per-second cinemavideo through 2-3 pulldown processing, wherein, when a detection resultindicates that the input video has been created through 2-3 pulldownprocessing, said control unit applies an identical gamma correctioncurve to output images created from an identical frame of the cinemavideo.
 9. An image processing method comprising the steps of: extractinga characteristic value of a luminance in relation to respective inputimages constituting an input video; detecting a presence of a scenechange in the input video; determining a gamma correction curve to beapplied to output images corresponding to the respective input images onthe basis of a magnitude of the extracted characteristic value; andcorrecting a luminance of the respective output images using thedetermined gamma correction curve, wherein, when a difference in thecharacteristic value between a first input image and a subsequent secondinput image is larger than a predetermined value and a scene change doesnot exist between the first input image and the second input image, agamma correction curve to be applied to the output image correspondingto the second input image is determined so as to suppress change of agamma curve in said determining step, and when the difference in thecharacteristic value between the first input image and the subsequentsecond input image is equal to or smaller than the predetermined value,the gamma correction curve corresponding to the characteristic value ofthe second input image is determined in said determining step as thegamma correction curve to be applied to the output image correspondingto the second input image.
 10. The image processing method according toclaim 9, wherein when the difference in the characteristic value betweenthe first input image and the subsequent second input image is largerthan the predetermined value and a scene change exists between the firstinput image and the second input image, the gamma correction curvecorresponding to the characteristic value of the second input image isdetermined in said determining step as the gamma correction curve to beapplied to the output image corresponding to the second input image. 11.The image processing method according to claim 9, wherein when thedifference in the characteristic value between the first input image andthe subsequent second input image is larger than a predetermined valueand a scene change does not exist between the first input image and thesecond input image, a gamma correction curve having a correctioncharacteristic between a gamma correction curve corresponding to acharacteristic value of the first input image and a gamma correctioncurve corresponding to a characteristic value of the second input imageis determined in said determining step as the gamma correction curve tobe applied to an output image corresponding to the second input image.12. The image processing method according to claim 11, wherein when thedifference in the characteristic value between the first input image andthe subsequent second input image is larger than a predetermined valueand a scene change does not exist between the first input image and thesecond input image, a gamma correction curve having an intermediatecorrection characteristic between a gamma correction curve correspondingto a characteristic value of the first input image and a gammacorrection curve corresponding to a characteristic value of the secondinput image is determined in said determining step as the gammacorrection curve to be applied to an output image corresponding to thesecond input image.
 13. The image processing method according to claim9, wherein the characteristic value is a total amount value of aluminance of a plurality of pixels included in the input image.
 14. Theimage processing method according to claim 9, further comprising a stepof inserting an interpolation image created using either a firstinterpolation method or a second interpolation method between adjacentinput images, wherein, in the first interpolation method, theinterpolation image is created by repeating an immediately precedingimage and in the second interpolation method, the interpolation image iscreated from preceding and following images, and said determining stepincludes determining a gamma correction curve to be applied to theinterpolation image in accordance with the interpolation method used tocreate the interpolation image.
 15. The image processing methodaccording to claim 14, wherein, when the interpolation image is createdusing the first interpolation method, said determining step includesapplying to the interpolation image an identical gamma correction curveto that of the immediately preceding image, and when the interpolationimage is created using the second interpolation method, said determiningstep includes determining the gamma correction curve to be applied tothe interpolation image on the basis of gamma correction curves appliedto the preceding and following images.
 16. The image processing methodaccording to claim 9, further comprising a pulldown signal detectionstep of determining whether or not the input video is a video createdfrom a 24 frame-per-second cinema video through 2-3 pulldown processing,wherein, when a detection result indicates that the input video has beencreated through 2-3 pulldown processing, said determining step includesapplying an identical gamma correction curve to output images createdfrom an identical frame of the cinema video.